This invention relates to a gas sensor of the type having an electrochemical gas-sensing element to which a plurality of lead wires are attached and a holder assembly including a rod-shaped holder of a heat-resistant and electrically insulating material formed with a plurality of axial holes through which the lead wires extend.
Currently various kinds of gas sensors are used in many fields concerned with high temperature gases such as combustion gases. For example, in the automobile industry it has been popular to use an oxygen sensor, or a carbon monoxide sensor, in an air-fuel ratio control system as a means for producing a feedback signal which is indicative of the concentration of oxygen, or carbon monoxide, in the exhaust gas and hence indicative of an actual air-fuel ratio of a gas mixture supplied to the engine. Oxygen sensors are also employed for controlling the air-fuel ratio in combustion apparatus other than automotive internal combustion engines and for controlling gas atmospheres in industrial furnaces.
Usually the sensitive part of currently used gas sensors is a gas sensing element that operates on an electrochemical principle. For example, an oxygen concentration cell utilizing an oxygen ion conductive solid electrolyte which serves as an oxygen sensing element, and an oxide semiconductor that undergoes a change in its resistivity with a change in the content of a specific substance in an ambient gas atmosphere is useful for producing an oxygen sensing element or a carbon monoxide sensing element. It is a recent trend to construct gas sensing elements in the form of a laminate of very thin layers on a substrate by using either a thin film technique or a thick film technique to thereby reduce the size of the element and enhance its sensitivity.
A practical gas sensor using a gas sensing element of the above described category is constructed so as to fixedly dispose the gas sensing element in a gas atmosphere subject to measurement, usually a hot gas atmosphere, and to measure the output of the sensing element at a location isolated from that gas atmosphere. Usually the gas sensing element is provided with a plurality of thin lead wires. In a typical sensor construction, the gas sensing element is fixed to one end of a rod-shaped holder which is made of a ceramic material such as alumina or mullite and has a plurality of axial holes. The lead wires are respectively passed through the axial holes in the ceramic holder, and the remaining spaces in the holes are filled with a heat-resistant sealant of which the principal component is usually an alumina powder. The ceramic holder is tightly fitted into a tubular metal casing, and the lead wires protruding from the free end of the ceramic holder are respectively connected to thicker wires which are passed through an elongate and axially bored plug formed of either a synthetic rubber or a synthetic resin. Usually the rubber or resin plug is held in axial alignment with the ceramic holder in the metal casing by tightly inserting this plug and an end portion of the metal casing into a metal sleeve.
In practice, however, it is very difficult to realize a truly hermetic seal in the elongate and cross-sectionally narrow holes in the ceramic holder. Besides, it is inevitable that the rubber or resin plug is slightly spaced from the end face of the ceramic holder. Therefore, during use of the gas sensor a small quantity of the gas subject to measurement such as an engine exhaust gas, passes through microscopic interstices in the ceramic sealant in the holes of the ceramic holder to reach the gap between the holder and the rubber or resin plug. Consequently, there occurs an accumulation of carbonaceous solid matter on the end faces of the holder and the plug, sufficient to cause the electrical insulation between the lead wires to lower. Moreover, the lead wires tend to gradually corrode. These phenomena become serious obstacles to accurate measurement of the output of the gas sensing element and significantly shorten the service life of the gas sensor.